Metal membranes that are selectively permeable to hydrogen are known. See, for example, U.S. Pat. Nos. 4,388,479 and 3,393,098, both of which disclose Group V and VIII alloy membranes such as palladium alloy catalytic membranes. The prohibitively high cost of palladium has lead to efforts to fabricate composite hydrogen-permeable metal membranes by coating certain transition metal alloy base metals with palladium or palladium alloys. See, for example, U.S. Pat. Nos. 4,468,235 and 3,350,846. The coating on such base metals imparts chemical resistance to the base metal and in some cases increases the rate of adsorption of hydrogen onto the metal membrane surface. However, such coated metal membranes have an inherent shortcoming in that, under the elevated temperature conditions of use, the coating metal tends to diffuse into the base metal, thereby destroying the benefits available from such composite metal membranes. U.S. Pat. No. 4,496,373 discloses a nonporous hydrogen-permeable composite metal membrane that addresses this intermetallic diffusion problem for a base metal alloy of a specific composition coated with a palladium alloy of specific composition. However, the composition of the palladium alloy coating and the base metal alloy are narrowly defined so as to favor partitioning of the palladium into the coating alloy as opposed to the base metal alloy. Consequently, this approach is not general in nature, requires strict control over alloy composition, and allows for little variation in selection of metals for membrane fabrication.
These and other shortcomings of prior art hydrogen-permeable composite metal membranes are overcome by the present invention, which is summarized and described in detail below.